Unbeknownst to him or his family IRA member Thomas McMahon had slipped onto the unguarded boat that night and attached a radio-controlled fifty-pound (23 kg) bomb.

Along with Lord Mountbatten those to die in the explosion were Nicholas Knatchbull, 14, one of his twin grandsons, and Paul Maxwell, 15, a local boat boy. Another passenger on the boat, the Dowager Lady Brabourne, 82, died the day after the attack.

The terrorist attack three decades ago led to one of the biggest police investigations in Irish history. Two men were charged: McMahon, then 31, and Francis McGirl, 24, a gravedigger.

At the time of the explosion, McMahon was 70 miles away, in police custody – by chance he and McGirl had been stopped at a checkpoint after he had laid the explosive.

One of the IRA’s most experienced bomb-makers, McMahon had flakes of green paint from Lord Mountbatten’s boat and traces of nitroglycerine on his clothes.

The bomb had been detonated by remote control at 11.39am when the boat was about 200 yards from the harbour.

The IRA quickly admitted carrying out the bombing saying it was designed to “bring to the attention of the English people the continuing occupation of our country."

On the day Mountbatten was assassinated the IRA ambushed and killed eighteen British Army soldiers, sixteen of them from the Parachute Regiment at Warrenpoint, County Down, in what became known as the Warrenpoint ambush.

Gerry Adams said of Mountbatten's death: "The IRA gave clear reasons for the execution. I think it is unfortunate that anyone has to be killed, but the furore created by Mountbatten's death showed up the hypocritical attitude of the media establishment.

"As a member of the House of Lords, Mountbatten was an emotional figure in both British and Irish politics. What the IRA did to him is what Mountbatten had been doing all his life to other people; and with his war record I don't think he could have objected to dying in what was clearly a war situation.

"He knew the danger involved in coming to this country. In my opinion, the IRA achieved its objective: people started paying attention to what was happening in Ireland."

Because there was insufficient evidence to place McGirl at the fishing village of Mullaghmore, he was acquitted and he died in 1995. McMahon was released from jail in August 1998 as part of the Good Friday peace agreement.

McMahon has never publicly discussed his role in the bombing. However, the year before his release from jail, his wife said: “Tommy never talks about Mountbatten, only the boys who died. He does have genuine remorse. Oh God yes.”

McMahon had served the first 13 years of his life sentence in the IRA wing of Portlaoise. He and ten others, armed with guns and explosives, failed in an attempt to escape in 1985.

Three years later, he fired a shot from a Browning pistol smuggled into a holding cell of Dublin’s Four Courts. But, in 1992, he claimed to have turned his back on the IRA.

After his release from prison McMahon helped with McGuinness' presidential campaign. He was been spotted putting up posters for Mr McGuinness around his home town of Carrickmacross, Co Monaghan.

He has never apologised for the Sligo murders and has refused to meet any of the victim's relatives.

Three people on the boat survived the attack on Mountbatten including Timothy Knatchbull, 14, – Nicholas’s twin brother – who blinded in one eye by the blast.

At Lord Mountbatten’s memorial service in December 1979, the Prince of Wales lashed out at “the kind of subhuman extremist that blows people up when he feels like it.” Prince Charles had always been close to his great uncle.

For decades the Queen was a prime target for the IRA.

There was also the plot to kill the Prince and Princess of Wales at a Duran Duran concert in London, and the attempt against Her Majesty and the Duke of Edinburgh on a trip to the province.